This invention relates to a process for coating substrates, in which a reaction gas capable of depositing a coating flows over the surface to be coated and the reaction gas is activated to a band-shaped plasma which is excited by microwaves generated in a waveguide. This invention also relates to a device in which the coating process of this invention and surface treatment of substrates with plasma can be performed.
To improve the coatability of a surface of various materials, plasmas generated by ultrahigh-, high-, or low-frequency discharges, as well as by direct current discharge, are often used. End use applications of this technique are cleaning or etching of surfaces, rendering plastic surfaces printable, e.g., by changing the wetting behavior thereof by liquids, and coating substrates with hard, scratch-resistant coatings or coatings that favorably change the optical and electrical properties of the substrate (interference coating systems, color coatings, electrically conducting coatings). For many uses, a high degree cf uniformity in treating or coating with respect to various properties, such as coating thickness, porosity, refractive index, and conductivity, is essential. This generally presupposes the use of a plasma that acts uniformly over the entire surface.
From DE-OS 3 830 249 there is known a plasma (CVD) process for coating large-area, planar substrates in which the plasma electrodes are placed in an array above or below the surfaces to be coated, and the distance between two adjacent plasma electrodes is of a size such that their plasma columns overlap one another. The drawback of this arrangement is that the microwave energies emitted by the antennas do not become superimposed in the plasma into a completely uniform energy input. Furthermore, interference effects of the waves radiated by the antennas can appear that also disrupt uniformity.
From DE-PS 3 147 986 there is known a device for generating a microwave plasma for treating substrates in which a waveguide structure is used that is designated a "slow wave structure." This arrangement also leads to a non-homogeneous band plasma because of the decreasing energy input into the plasma along the waveguide structure.
From M. Geisler, J. Kieser, E. Raeuchle, R. Wilhelm, Journal of Vacuum Science and Technology A8, 908 (1990) "Elongated microwave electron cyclotron resonance heating plasma source," there is known a band-shaped arrangement (ECR) of permanent magnets and a microwave feed by elongated horn antennas to generate a band-shaped plasma.
While the relative movement of substrate and bandshaped plasma yields a good, homogenizing treatment and coating in the direction of movement, in all variants the homogeneity of the plasma in the direction of the band appears insufficient. Even with an offset arrangement of two such antennas, the one-dimensional grid of the slow wave structure does not lead to a completely uniform energy input into the plasma.
In addition to the same concerns as with the slowwave structure, the inherent drawback of the ECR arrangement is the necessary low process pressure. If it is desired to achieve a high coating rate, which is generally the case, vacuum pumps with great pumping speed must be used to be able to pump out a large mass flow at a low process pressure. This represents an undesirably high equipment expense. Also, the requirements on the leak rate of load lock systems increase. Another drawback is in that long lead times are required to achieve the necessary operating pressure.
From DE-OS 3 926 023 there is known a CVD coating process for the production of coatings and a device for performing the process in which an antechamber in the form of a metallic tube with a slit-shaped opening is placed above the reaction space in which the substrate to be coated is positioned. In this antechamber, microwaves are generated that excite a plasma in the antechamber. The excitation of the reaction gas introduced into the reaction space is performed then by excited species that flow out of the plasma into the reaction space. Thus the plasma is located mostly in the antechamber and not in the reaction space. To coat a large-area substrate, the substrate is moved perpendicularly to a slit-shaped opening of the antechamber. The drawback of this process is that, because of the indirect excitation, only low coating and reaction rates can be achieved. This process is less well suited, e.g., for producing fully oxidized and impenetrable coatings of low porosity.
It is an object of this invention to provide a process and a device with which large-area substrates can economically and, in particular, in a short time, be provided with coatings which are more impenetrable and more homogeneous than the coatings that can be achieved with CVD plasma coating processes according to the prior art.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.